1. Field of the Invention
This invention relates to a novel pressure-sensitive adhesive composition, and more particularly to a pressure-sensitive adhesive composition which is water-soluble and which shows adhesive properties upon the application of pressure under any conditions.
2. Description of the Prior Art
Water-soluble adhesives are known in the art; for example, a so-called "gum tape", comprising a paper substrate with glue, dextrin, or polyvinyl alcohol coated thereon, is well known. Under dry conditions, however, this type of tape lacks rubber elasticity and viscosity, showing no pressure-sensitive adhesive properties. Rather such a tape shows pressure-sensitive adhesive properties only when water is applied thereon. Thus conventional water-soluble adhesives have disadvantages in that they need a complicated wetting apparatus when used for industrial applications, and when they are used on stamps and envelopes, difficulties are also encountered in wetting and applying them.
More recently, various kinds of water-soluble or water-dispersible adhesives avoiding the above described disadvantages and showing pressure-sensitive adhesive properties under various conditions have been developed. For example, U.S. Pat. No. 2,838,421, Japanese Patent Publication Nos. 34578/1975, 22768/1977 and 23813/1974 disclose such pressure-sensitive adhesives comprising a polymer component, such as a polymer containing specific carboxyl groups or a water-soluble polymer salt thereof, e.g., an alkali metal salt, an amine salt or an alkanolamine salt, and a water-soluble or water-dispersible plasticizer, such as polyether polyol, a polyhydric alcohol or an alkanolamine salt of acidic rosin.
U.S. Pat. No. 3,096,202 discloses a pressure-sensitive adhesive wherein a polyvinyl-pyrrolidone elastomer is used as the polymer component, and to this polymer component, a water-soluble plasticizer, such as polyether polyol, is added in combination with a polyfunctional polymerizable cross-linking agent, such as ethyleneglycol dimethacrylate, or an activation catalyst.
These water-soluble or water-dispersible pressure-sensitive adhesives have advantages in that they will easily adhere by the application of nothing more than manual pressure, and in that they provide strong adhesive strength immediately after application.
In this type of conventional pressure-sensitive adhesive, as described above, the component employed as the polymer has a very high polarity, and a glass transition temperature higher than room temperature, such as a carboxyl group-containing polymer or polyvinyl-pyrrolidone, in order to provide the function of water solubility, and in order to plasticize such a polymer to provide it with pressure-sensitive adhesive properties, a large amount of a water-soluble or water-dispersible liquid plasticizer having a relatively low molecular weight is incorporated. A particular problem encountered with these pressure-sensitive adhesives is poor compatibility of the polymer component with the water-soluble or water-dispersible plasticizer, and when they are stored for a long period of time, the plasticizer begins to migrate (also referred to as bleeding). When a paper material is employed as a substrate on which the adhesive is applied, the bleeding is accelerated by the affinity between the paper material and the water-soluble or water-dispersible plasticizer.
Therefore, when this type of adhesive is used, for example, in bonding original papers together or adhering pieces of paper in the paper industry, the plasticizer in the adhesives easily migrates into the paper, deteriorating the quality of the print being applied on the paper.
Water-soluble or water-dispersible pressure-sensitive adhesive have been developed as an adhesive suitable for use in the above-described applications in the paper industry. The reason for this is that the pressure-sensitive adhesive characteristics improve the adhesion, while at the same time, the water-soluble or water-dispersible characteristics render the paper easily soluble or dispersible in water upon repulping of the paper.
However, the bad print resulting from bleeding of the plasticizer which has been encountered with the conventional adhesives is a serious hindrance to the use of the adhesives in the above-described applications, greatly reducing the value of such adhesives in water-soluble or water-dispersible pressure-sensitive adhesives.
Recently it has been reported in, for example, U.S. Pat. No. 3,891,584 that graft copolymers produced by addition-polymerization of an unsaturated monomer, such as vinyl acetate, vinyl propionate or vinyl butyrate, with a high molecular weight water-soluble polymer, such as polyethylene glycol having a molecular weight of about 3,000 to about 20,000, can be used in providing an adhesive with suitable water-dispersion characteristics.
In producing these graft copolymers from a water-soluble polymer and an unsaturated monomer, a bulk polymerization method is employed in copolymerizing the unsaturated monomer, in order to minimize the formation of homopolymers, and the graft copolymers so formed are high molecular weight polymers in the gel form. Therefore, although these graft copolymers can be compounded with rosin or its derivative, or a viscous resin such as a cumarone-indene resin to obtain hot melt adhesives, they cannot be employed at all for the production of the pressure-sensitive adhesive of this invention. In addition, the water-dispersion characteristics obtained by the choice of the unsaturated monomer are unsatisfactory.